1. Field of the Invention
Azithromycin is the USAN generic name of the azalide 9-deoxo-9a-aza-9a-methyl-9a-homoerythromycin A, which systematic name is 1-oxa-6-azacyclopentadecan-15-one, 13-((2,6-dideoxy-3-C-methyl-1-3-O-methyl-alpha-L-ribo-hexopyranosyl)-oxy)-2-10 ethyl-3,4,1 0-trihydroxy-3,5,6,8,10,12,14-heptamethyl-11-((3,4,6-trideoxy-3-(dimethyl-amino)-beta-D-xyl It is a semisynthetic macrolide that shows an excellent antimicrobial activity against gram-positive and some cases of gram-negative bacteria (H. A. Kirst, G. D. Sides, Antimicrob. Agents. Chemother. 1989, 33, 1419-1422). Clinical use of this macrolide is broadening its application to the treatment of opportunistic infections (F. Lecomte, Rev. Med Interne 1998, 19(4), 255-61; S. Alvarez-Elcoro, Mayo Clin. Proc. 1999, 74(6), 613-34; J. Schater, Lancet, 1999, 354(9179), 630-35).
2. Description of the Prior Art
FIG. 1 shows the different synthetic routes to azithromycin 1. The names of the intermediates displayed in FIG. 1 are gathered in the following table.
The following table summarizes the patents, articles, authors and applicants that describe the different synthetic paths (A, B, C, D, E) towards azithromycin 1.
The structural elucidation studies carried out with azithromycin 1 have shown the existence of two different crystalline forms: hygroscopic monohydrate and non-hygroscopic dihydrate, being the latter preferred for manufacturing formulations used in therapeutical treatments, as it is described in EP 0,298,650.
Azithromycin dihydrate is easily distinguishable from hygroscopic azithromycin by means of the following differentiative assays:
a) The dihydrate form keeps its percentile water content constant at values (4.5-5%) which are very close to the theoretical value (4.6%).
b) The differential calorimetry analysis (DSC) of azithromycin dihydrate reveals the presence of a single endotherm which may vary between 115 and 135xc2x0 C., with an energy absorbed during the process which ranges between 27 and 34 cal/g.
c) Each crystalline form presents its own characteristic X-Ray Diffraction spectrum
d) The infrared spectra in KBr of both crystalline forms present clear differences:
Two other synthesis, affording azithromycin 1 as a form that should differ from the crystalline ones previously mentioned, have also been described. In these cases, azithromycin is obtained by simple evaporation to dryness. However, in these documents there is no reference to the crystalline state of the azithromycin thus obtained.
In the following table are summarized the different procedures for the preparation of both crystalline forms of azithromycin 1.